Apparatus for successively heating long moving pieces of conductive materials



Dec. 6, 1966 A. HINKEL 3, 0,

APPARATUS FOR SUCCESSIVELY HEATING LONG MOVING PIECES OF CONDUCTIVE MATERIALS Filed May 26, 1965 9 R L S r J Inventor:

Arfur HINKEL A Via KM 0. m

his Afiorney United States Patent K 5 Claims. (Cl. 219-155) The present invention relates to a heating apparatus for successively heating a long piece of an electrically conductive material, especially steel, while this material is traveling in its longitudinal direction by passing an electric current in this direction through the material and thereby heating it by its electrical resistance.

The heating apparatus which are known for this purpose are generally provided with three heating circuits or areas which are located behind each other in the longitudinal direction of the material and each of which is provided with two sets of contact rollers which are applied behind and spaced from each other upon the material and are connected to a three-phase alternating current supply line in such a manner that one conductor of each heating circuit is connected to a different phase of the supply line in order to apply the load thereon as uniformly as possible.

These known heating apparatus are, however, unsuitable for heating a material of a large cross-sectional area, for example, pipes which have a considerable weight per unit of length, since the electric efficiency of such an apparatus is in such a case very poor and it is therefore uneconomical to heat the material by using its electrical resistance. This is due to the fact that the larger the weight of material of one of the conventional cross-sectional shapes might be per unit of length, the larger should also be the contact surface of the current-supplying contact rollers. However, this is practically impossible since the electric load which may be applied on the contact surface of these rollers is limited. Consequently, the larger the cross-sectional area of the material which is to be heated, the smaller is the heating effect which can be attained per unit of length of the material. In order to heat the material to a certain temperature, it is therefore necessary to keep the material within the areas of the individual heating circuits for such a length of time as its crosssectional area requires. However, since the feeding speed at which the material passes through the heating apparatus usually cannot be varied since it is generally determined by the speed at which it travels through other apparatus, for example, a rolling mill, it is only possible to attain a greater length of time during which a material of a larger cross-sectional area can be heated so as to reach the desired temperature, by increasing the length of the individual heating areas, that is, the distance between the adjacent sets of contact rollers. However, since the resistance heating of the material is produced by an alternating current, such an increase of this heating distance also means an increase of the inductive voltage drop and thus an increase of the requirements of the reactive portion in kilovolt-amperes, i.e. of the wattless power output. This increase and the resulting higher losses in the transformer which supplies the heating current to the contact rollers increase the cost of the heating operation.

When heating long elements of steel, for example, pipes, an additional factor occurs namely, since the specific resistance of steel varies very considerably with its temperature, the heating distances between the adjacent sets of contact rollers must differ from each other. When heating such an element, for example, to about 900 C. within three heating areas in each of which substantially 3,290,481 Patented Dec. 6, 1966 the same amount of energy is supplied to this element, the

first two sets of rollers must be spaced from each other at twice the distance at which the two following sets of rollers are spaced from each other. On the other hand, the length of the central heating area only needs to be slightly larger than that of the last heating area. This result-s in an unsymmetrical load upon the phases of the alternating supply current which, in turn, has a negative effect upon the economy of operation of the heating apparatus.

It is an object of the present invention to provide a heating apparatus which may be operated economically and is particularly adapted for heating a material of a large cross-sectional area which requires relatively long heating areas. According to the invention, this object is attained by connecting at least one heating circuit which is supplied with direct current by means of a rectifier in series with the heating circuits which are supplied with alternating current.

The degree of efiiciency of such a direct-current heating circuit is independent of the length of the distanceof its heating area. It therefore permits the material to be heated to such an extent that the following heating areas, the circuits of which are supplied with alternating current, may be made of a length which does not considerably reduce the efficiency of the apparatus. If the element to be heated consists, for example, of steel, this element is preferably heated within the direct current circuit to ap proximately 400 C. In order to make the total length of the entire heating apparatus as small as possible, it may, however, be more advisable to heat the material within the direct-current circuit to a lower temperature. The areas which are heated by the alternating current circuits then have to be made of a greater length in order to compensate for the shorter length of heating the material within the direct current circuit. However, this increase in length of the alternating current circuit heating means may be smaller than a reduction of the direct current circuit heating means which might be attained if the ma terial is heated for a shorter length of time within the shorter area of the direct current circuit.

The heating circuit which is to be supplied with direct current may be connected to a rectifier which, in turn, may be connected to the same polyphase current supply line to which the other heating circuits are connected, arranged so that each phase of this line is loaded by a different heating circuit.

According to a preferred embodiment of the invention, the heating circuit which is to be supplied with direct current is connected in series with a number of alternating cur-rent heating circuits which equals the number of phases of the alternating current supply line. A substantially symmetrical load upon all phases of the supply line may then be attained if the rectifier for supplying the direct current heating circuit is connected to all phases of the supply line and adapted to apply an equal load upon all of these phases.

It should be noted that the specific electric resistivity of steel in particular increases substantially with an increase of its temperature. Since the heat energy produced over a particular length of, for example, steel pipe is equal to the product of the ohmic resistance thereof times the square of the electric current intensity passing there through, the heat produced and consequently the load over successive segments of equal length of steel pipe are not equal. In the prior art these successive segments were generally fed from a three-phase alternating current source via a transformer. In order to make use of as simple a transformer as possible and to load the circuit symmetrically, the prior art devices used generally a polyphase transformer, each phase of which supplied current to a segment of the pipe. Due to the difference in load circuit depends. of the material 6, due to its increased temperature, will factors for each segment of pipe to be heated the threephase alternating current source is unequally loaded which is, as explained above, undesirable. It is, of course, possible to make the heat energy produced in each segment of pipe equal by varying the pipe segments accordingly. This would, however, result in pipe segments, and consequently heating means, of radically different length which is also uneconomical and undesirable.

The device of this invention overcomes to a certain .degree all of the afore-described difiiculties and disadvantages of the prior art by providing first longitudinally disposed direct current heating means and then at least two longitudinally disposed alternating current heating means, all of which are connected to a three-phase alternating current source via a plurality of transformers. Such an arrangement causes surprisingly a much more symmetrical load distribution on the three-phase alternating current source and also makes for a more compact overall construction.

These and other features and advantages of the present invention will become further apparent from the following detailed description thereof which is to be read with 1 reference to the accompanying drawings, in which:

FIGURE 1 shows a circuit diagram of a heating apparatus according to a first embodiment of the invention, while FIGURE 2 shows a circuit diagram of a heating ap paratus according to a second embodiment of the invention.

The heating apparatus according to the invention as illustrated in FIGURE 1 comprises five sets of contact rollers 1 to 5 which are placed behind and at certain distances from each other upon a long piece 6 of an electrically conductive material, for example, a steel pipe, which is to be heated while traveling at a certain speed in the direction as shown by the arrow A. The two sets of rollers 1 and 2 which form the electrodes of the first heating circuit or heating stage are connected to a rectifier 7 which, in turn, is connected to the secondary winding of a transformer 9, the primarywinding of which is connected to the two phases S and T of a three-phase alternating current line 8, the third phase of which is designated by the letter R. While traveling in the direction A, the material 6 is therefore successively heated by a rectified current flowing in the longitudinal direction through the material between the two sets of contact rollers 1 and 2. The particular distance at which these two sets of rollers 1 and 2 are to be separated from each other depends upon the resistance of the material between these two sets, the admissible contact load of the rollers,

and the height of the voltage which may be applied.

The subsequent sets of contact rollers 3 and 4 form the electrodes of the second heating circuit or heating stage. They are connected to the secondary winding 10' .of a transformer 10, the primary winding of which is connected to the phases S and R of the three-phase cur rent line 8. After being heated by a direct current in the first heating circuit, the material 6 is therefore further heated by a flow of alternating current between the two sets of contact rollers 3 and 4. The distance between these two sets depends upon the same factors upon which the distance between the rollers 1 and 2 of the first heating However, since the specific resistance generally be higher between the rollers 3 and 4 of the second heating circuit than that between the rollers 1 and 2 of the first circuit, the distance between the rollers 3 and 4 may normally be made shorter than that between 'winding 11' of a transformer 11, the primary winding of which is connected to the phases R and T of the alternating current line 8. The material 6 is therefore likewise heated by a flow of alternating current between the rollers 4 and 5.

In place of three separate transformers 9, 10, and 11, it is also possible to employ a single three-phase transformer (not illustrated). In both cases it is advisable to maintain the associated sets of contact rollers at such distances from each other that the amount of energy which is required for each of the three heating units will be as equal as possible so that the load will be distributed as symmetrically as possible between the three phases of the supply line 0.

However, due to the different ratio between the watt component and the wattless component of the power output in the different heating circuits, the apparatus according to FIGURE 1 does not permit the load upon the supply line 8 to be as symmetrical as desired. This may, however, be attained by modifying the apparatus accord ing to the invention in the manner as illustrated in FIG- URE 2.

This heating apparatus according to FIGURE 2 com prises six sets of contact rollers 101 to 106 of an e1ectri= cally conductive material which are applied behind each other on a long piece of material 107 of an electrically conductive material which travels at a certain rate of speed in the direction of the arrow B. The twos'ets of rollers 101 and 102 are connected to a rectifier 108 which is supplied with alternating current by a three-phase trans= former 110 which is connected to the three phases, R, S, and T of the alternating current supply line 109. The rectifier 108 is of a conventional construction and designed so as to load the three-phase transformer 110 symmetrically. The distance between the sets of rollers 101 and 102 again depends upon the resistance of the material between these rollers, upon the admissible contact load, the height of the voltage which may be applied, the heat to be produced in the material, and the admissible length of the entire apparatus.

The sets of contact rollers 103 to 106, which are mounted longitudinally subsequent to the sets 101 and 102, form the electrodes of three further heating circuits, each of which comprises two sets of contact rollers which are supplied with alternating current by a separate trans formed 111, 112, and 113, respectively. The transformer 111 is connected to the phases T and S, the transformer 112 to the phases S and R, and the transformer 113 to the phases R and T of the supply line 109. If the heating circuits which are supplied with alternating current are made of such a length that the energy consumption of all three of them will be substantially equal, the total load on the three phases of the supply line will be substantially uniform since the load of the direct current circuit is likewise uniformly divided over all three phases. This em bodiment of the invention has the further advantage that the distance between the contact rollers 101 and 102 of the direct current heating circuit may be made of any desired length independently of the distance between the rollers of the alternating current heating circuits. This facilitates the adjustment and operation of a relatively short length apparatus.

In place of the three transformers 111 to 113 it is also possible to employ a single three-phase transformer (not illustrated) which may be connected in a suitable manner so as to balance any difierences in the load which is applied upon the three phases of its secondary side.

Although my invention has been illustrated and described with reference to the preferred embodiments thereof, I wish to have it understood that it is in no way limited to the details of such embodiments but is capable of numerous modifications within the scope of the appiece of an electrically conductive material by using the placed downstream of said first circuit in the direction of 5 travel of said work piece, means for supplying an alternating current to each of said further heating circuits, each of said heating circuits comprising at least two contact rollers substantially parallel to and spaced from each other and each adapted to rotate about an axis extending transverse to the longitudinal direction of movement of said work piece to be heated, all of said contact rollers being adapted to be applied upon said work piece in a row behind each other in said longitudinal direction of movement for conducting said direct and alternating currents separately and successively between said two rollers of each circuit through said moving work piece in said longitudinal direction of movement thereof.

2. A heating apparatus as defined in claim 1, in which said alternating current supply means comprise a poly- 20 phase current supply line, said direct current supply means comprising a rectifier also adapted to be connected to said polyphase current supply line.

3. A heating apparatus as defined in claim 2, in which one conductor of each of said circuits is adapted to be connected to a different phase of said polyphase supply line.

4. A heating apparatus as defined in claim 2, in which said rectifier is adapted to be connected to all phases of said polyphase supply line to which said further heating circuits are adapted to be connected.

5. A heating apparatus as defined in claim 4, in which said rectifier is adapted to apply a substantially equal load to all of said phases of said polyphase supply line to which it is adapted to be connected.

References Cited by the Examiner UNITED STATES PATENTS 2,034,411 3/1936 Mitchell 219l08 2,164,850 7/1939 Wood et al. 2663 X FOREIGN PATENTS 547,035 3/1932 Germany.

RICHARD M. WOOD, Primary Examiner. ANTHONY BARTIS, Examiner.

B. A. STEIN, Assistant Examiner. 

1. AN APPARATUS FOR SUCCESSIVELY HEATING A LONG WORK PIECE OF AN ELECTRICALLY CONDUCTIVE MATERIAL BY USING THE ELECTRIC RESISTANCE THEREOF WHILE SAID WORK PIECE IS MOVING IN ITS LONGITUDINAL DIRECTION, COMPRESING AT LEAST ONE FIRST HEATING CIRCUIT, MEANS FOR SUPPLYING A DIRECT CURRENT TO SAID FIRST CIRCUIT, AT LEAST TWO FURTHER HEATING CIRCUITS, DISPLACED DOWNSTREAM OF SAID FIRST CIRCUIT IN THE DIRECTION OF TRAVEL OF SAID WORK PIECE, MEANS FOR SUPPLYING AN ALTERNATING CURRENT TO EACH OF SAID FURTHER HEATING CIRCUITS, EACH OF SAID HEATING CIRCUITS COMPRISING AT LEAST TWO CONTACT ROLLERS SUBSTANTIALLY PARALLEL TO AND SPACED FROM EACH OTHER AND EACH ADAPTED TO ROTATE ABOUT AN AXIS EXTENDING TRANSVERSE TO THE LONGITUDINAL DIRECTION OF MOVEMENT OF SAID WORK PIECE TO BE HEATED, ALL OF SAID CONTACT ROLLERS BEING ADAPTED TO BE APPLIED UPON SAID WORK PIECE IN A ROW BEHIND EACH OTHER IN SAID LONGITUDINAL DIRECTION OF MOVEMENT FOR CONDUCTING SAID DIRECT AND ALTERNATING CURRENTS SEPARATELY AND SUCCESSIVELY BETWEEN SAID TWO ROLLERS OF EACH CIRCUIT THROUGH SAID MOVING WORK PIECE IN SAID LONGITUDINAL DIRECTION OF MOVEMENT THEREOF. 